Recent power electronics circuits achieve power conversion by switching semiconductor devices. Although such a scheme has an advantageous feature of highly efficient power conversion, large electromagnetic noises are generated because of its operating principle, i.e., sharp changes in voltage and/or current. A large noise filter may be required to reduce noise transmitted from the circuit, and such a filter prevents an apparatus from being reduced in size.
According to conventional noise filters, common mode choke coils and Y capacitors (Y condensers) are used as countermeasure components for reducing common mode noise. However, in the power electronics circuit, there is a problem that capacitances of the Y capacitors are limited by regulations on a leakage current, and the common mode choke coils are increased in size in order to avoid magnetic saturation and reduce copper loss.
When reducing the size of a noise filter, since there is a limit in optimization of the filter itself, it is important to reduce an amount of noise being generated in a circuit as a noise source. In particular, in the power electronics circuits, since the common mode noise is dominant in frequency bands of 1 MHz or higher, it is important to reduce the common mode noise in a frequency band of 1 MHz to 100 MHz (hereinafter, referred to as “MHz band”).
As circuits for reducing the common mode noise, there are, e.g., inventions disclosed in Patent Documents 1 and 2.
According to Patent Document 1, two terminals of a switching element as a noise source are connected to a power source via inductive lines on a substrate, and further connected to a metal housing through parasitic capacitances appearing between the lines and the housing. These noise sources, inductances, and parasitic capacitances constitute a bridge circuit. Patent Document 1 discloses reducing generation of a common mode noise by configuring the substrate so as to satisfy a certain relationship among the inductances and the parasitic capacitances.
In addition, Patent Document 2 discloses reducing a noise flowing out of a power supply circuit, by configuring a part of the power supply circuit, which is an insulated buck DC-DC converter, in a manner similar to the bridge circuit of Patent Document 1.